As modern electronic devices, especially integrated circuits (ICs), become more complex there is a great need to extend circuit integration into three dimensions. This is especially true of devices and circuits that operate at high frequencies where there is often a need to include integrated passive devices (e.g., inductors, capacitors, resistors, transmission lines, ground planes, shielding structures, baluns, etc.) that cannot easily be provided as a part of the associated semiconductor devices. Accordingly, such integrated passive devices (IPDs) are often formed in dielectric and metal layers above the semiconductor substrate in or on which the active devices, e.g., transistors of various kinds, are formed. (As used herein, the term “transistor” singular or plural, is intended to include any type of semiconductor device having two or more terminals.) The greater the number and complexity of the integrated passive devices (IPDs), the greater the need to extend the integrated circuit structure into the third dimension perpendicular to the surface of the underlying semiconductor devices. Such devices and circuits are referred to as “3-D integrated circuits” or “3-D ICs”.
Creating effective 3-D ICs incorporating high frequency power amplifiers has proved especially difficult because of electromagnetic (EM) cross-talk among the various components and higher than desired losses arising from stray electromagnetic (EM) fields inducing undesirable eddy currents in underlying semiconductor substrates. These effects can limit the gain and efficiency of high frequency power amplifiers. These effects are especially pronounced with advanced LDMOS (laterally diffused metal oxide semiconductor) integrated power amplifiers that employ high resistivity (e.g., semi-insulating) substrates. The thicker the substrate the greater the decoupling and the higher the quality factor Q of the associated integrated passive devices (IPDs). The quality factor Q is a measure of the energy stored divided by the energy dissipated per cycle by a resonant element, such as for example (but not limited to) an inductor. However, use of thicker substrates creates other problems, such as for example, increased thermal impedance between power amplifier active device (AD) regions on or near a front face of the substrate and a heat sink coupled to a rear face of the substrate. This increased thermal impedance can degrade overall performance. Thus, power amplifier ICs embodying IPDs involve conflicting requirements. For example, active device (AD) performance is generally optimized by using thinner substrates for efficient heat extraction, while integrated passive device (IPD) performance is generally optimized by using thicker substrates. 3-D integration attempts to avoid this conflict by moving the IPDs to layers above the active devices. However, there are physical limits on the number and thickness of multilayer dielectric-metal stacks for IPDs that can be deposited on a semiconductor substrate containing active devices (ADs). This can make it difficult or impossible, for example, to reduce the cross-talk among the IPDs and/or between the IPDs and the underlying ADs and their substrate. Thus, a need continues to exist for improved 3-D IC structures and methods where undesirable electromagnetic cross-talk and thermal impedance effects are simultaneously minimized or avoided. This is especially true in the case of high frequency power amplifiers where cross-talk, thermal impedance and other present day limitations are acutely felt.